The present invention relates to an adjustable metering valve for an internal combustion engine fuel injector, and to the relative method of adjustment.
A metering valve is normally controlled by the armature of an electromagnet, and is fitted to the body of the injector. The travel or lift of the armature towards the electromagnet core affects discharge of the injector, while the gap between the armature and the core affects response of the valve when the electromagnet is de-energized. As such, both the travel and gap must be set accurately.
Various metering valves are known in which the armature is connected to a stem guided by a sleeve with a stop flange; and the travel of the armature is defined by arrest of the flange against an edge of the sleeve. In one known metering valve, the sleeve is fitted inside the injector body with the interposition of a shim, and the electromagnet is fitted to the injector body by means of a jacket with the interposition of a second shim. In another known metering valve, the guide sleeve flange is fitted between a shoulder of the sleeve and an edge of the electromagnet jacket with the interposition of two groups of shims.
In both cases, the two shims are selected from a number of calibrated shims of modular thicknesses differing by a very small amount, which, for technical reasons, cannot be less than the machining tolerances, e.g. five micron. Adjusting the travel of the armature to a 5 micron tolerance, however, is often not accurate enough to keep discharge of the injector within the narrow range demanded by modern, particularly high-speed, internal combustion engines.